1. Field of the Invention
The present invention relates to a humidity sensitive device and humidity sensitive apparatus for use in detection of humidity in food cooking apparatus, dryer, air conditioner or measurement apparatus.
2. Description of the Prior Art
Generally speaking, metal oxides are superior in physisorption of water, and the amount of the physisorpted water depends on temperature and humidity of the atmosphere. When water is physisorpted on the surface of the metal oxide, ionic conduction will then increase thereby lowering the oxide's electrical resistance.
Most conventional humidity sensing devices utilize physisorption of water molecules on the surface of the above-mentioned metal oxide. Metal oxides such as Fe.sub.3 O.sub.4, Fe.sub.2 O.sub.3, Cr.sub.2 O.sub.3, Ni.sub.2 O.sub.3, Al.sub.2 O.sub.3, TiO.sub.2, ZnO, MgCr.sub.2 O.sub.4, TiO.sub.2 -V.sub.2 O.sub.5 compositions have been conventionally utilized and ZnO-Cr.sub.2 O.sub.3 -Li.sub.2 O-V.sub.2 O.sub.5 compositions.
The conventional humidity sensitive device is capable of converting a humidity value an electrical resistance value, and thus by utilizing this characteristic various humidity sensing apparatuses have been devised.
Known uses for humidity sensing devices include air conditioners, food cooking apparatus, dryers and measurement apparatus.
However, the above-mentioned conventional humidity sensing device has the following problem. That is, the conventional humidity sensing device has a poor humidity-resistance characteristic against severe atmosphere and is thus limited to atmospheric temperature environments for humidity detection. The reason for the poor characteristic is that when dust or oil vapor is physisorpted onto the metal oxide surface, the physisorption of water changes. The reason for narrow atmospheric temperature detectability of humidity is that, above 100.degree. C. the physisorption of water onto the metal oxide surface rapidly decreases and under the atmospheric temperature of 0.degree. C., the water physisorpted on the metal oxide surface physically changes to an ice state.
In order to improve the above-mentioned atmospheric humidity-resistance characteristic, the sensitive device has previously been washed by organic solvent or heated under high temperatures to burn off the physisorpted substances. However, such conventional methods hinder continuous detection of humidity.
The above-mentioned shortcoming of the humidity sensitive device is a common problem in any device utilizing the principal of water physisorption.
Another type of atmosphere sensitive device is known however which does not utilize the physisorption of water. This known device detects gas by utilizing ceramic of SnO.sub.2 composition. However, this conventional gas detection device comprising the SnO.sub.2 ceramic detects reducing gas, and therefore when reducing gas exists in addition to humidity, then the sensitive device exhibits a considerable output. Accordingly, such a sensitive device cannot be used for accurate measurement of humidity alone in an atmosphere which may include reducing gas.
From the above-mentioned circumstancess there has been a great demand need for humidity sensing device which is sensitive only for a humidity, apart from reducing gas, in a wide range of atmospheric temperatures.